1. Technical Field
The present invention relates to an image data creation apparatus, an image processing apparatus, an image processing system, an image data creation method, an image processing method, an image data creation program and an image processing program which perform processing for conversion between color image data and black-and-white image data.
2. Related Art
Heretofore, in order to convert color image data to black-and-white data, there has been widely used a method of calculating gray values from corresponding RGB values of color image data in accordance with a formula (1) shown below to obtain black-and-white data represented by the gray values. Since the gray values reflect corresponding pieces of brightness information related to an original color image, this method enables obtaining of image data corresponding to a black-and-white image reflecting the brightness levels of the original color image. In addition, in the formula (1), “R”, “G” and “B” denote a gray-scale value for red, a gray-scale value for green and a gray-scale value for blue, respectively, and “Gray” denotes a gray value, that is, a density level of a single black color.Gray=0.3R+0.59G+0.11B  (1)
Further, in the case where color image data is converted to black-and-white data in accordance with the formula (1), as a result, a plurality of RGB values, such as (R, G, B)=(255, 0, 0), (0, 128, 0), is converted into corresponding gray values which are approximately equal to one another. As described above, a plurality of colors having been different from one other in a color image before conversion result in gray colors having substantially the same gray value in a black-and-white image after conversion, thereby, sometimes, making it difficult to distinguish colors of an original color image before conversion from a black-and-white image after conversion.
In order to overcome this disadvantage, in JP-A-2011-23895, there has been proposed a method for converting a color image into a black-and-white image in accordance with the following processes: converting an original color image into color images which are represented in cyan, magenta and yellow, respectively; applying a dither matrix to the individual color images to create different binary dot patterns for the respective colors; and combining the resultant binary dot patterns for the respective colors to obtain a target black-and-white image. According to this method disclosed in JP-A-2011-23895, it is possible to obtain a black-and white image resulting from superimposing mutually different patterns which are represented in colors of cyan, magenta and yellow, respectively. Thus, even in the case where, for example, different colors (R, G, B)=(255, 0, 0), (0, 128, 0) and the like, which result in having substantially the same gray value after having been converted into gray values, are used in an original color image, the resultant black-and-white image reflects the different colors used in the original color image as corresponding features having mutually different patterns. Accordingly, even though colors resulting in being indistinguishable after having been converted into gray values are used in an original color image before the conversion, it is possible to convert image data corresponding to the original color image into image data corresponding to a black-and-white image from which the differences of colors in the original color image can be distinguished.
In the method disclosed in JP-A-2011-23895, however, although it is possible for users to distinguish the differences of colors in an original color image from a resultant black-and-white image, it is difficult to restore the original color image from the black-and-white image because it is difficult to specifically find out which one of the colors each feature of the black-and-white image corresponds to.